A variety of voltage regulators have been developed to provide supply voltages to digital and analog electronic systems with desirable tolerances. Typically, high performance digital electronic systems for computer, communication and industrial applications include many integrated circuits (e.g., microprocessors, digital signal processors, driver circuits, memory, etc.). The processing power, clock frequency, size and power consumption of such semiconductor devices are continually increasing. Corresponding improvements and more precise control of input voltage and current are usually needed to achieve desired improvements in performance of such semiconductor devices.
To control input voltages, an integrated circuit (IC) employs a voltage regulator to maintain a desired constant input voltage at the input to the IC. In addition to maintaining a substantially constant input voltage during normal (e.g., steady-state) operation of the IC, the electrical characteristics during start-up of the circuitry also can affect integrity and performance of the IC. Accordingly, dual mode voltage regulators have been developed in an effort to improve performance characteristics during start-up and normal operation.
A dual mode voltage regulator can operate in a start-up mode and in a normal mode. In the start-up mode, circuitry is provided to slowly ramp up (or down) to a desired input voltage in an effort to avoid overshoot that otherwise might occur. For example, one type of dual-mode voltage regulator uses current sources to charge an associated capacitor to a desired voltage for associated IC circuitry over a brief period of time, called a start-up phase. After completion of the start-up phase, the voltage regulator operates in a normal operating mode in which it provides a second fixed voltage (the operational voltage of the IC) on the input to the IC.
Unfortunately, the performance of prior art voltage regulators, including dual-mode voltage regulators, can often be unsatisfactory and thus suffer undesirable effects. For example, even small sudden changes in the input voltage can inject noise into the IC circuitry. In addition, though dual-mode voltage regulators reduce the damage caused by placing an operational voltage source directly on an IC circuitry, over-voltage conditions (e.g., spikes) may still occur. However, conventional dual-mode voltage regulators implemented in the IC tend to occupy a relatively large amount of IC wafer space. Existing dual-mode solutions that require circuitry external to the IC usually require an extra pin to electrically couple the external circuitry with the IC. Such external compensation systems further may require a large compensation capacitor to reduce slew rate, which increases the cost of the resulting circuitry.